Field of the Invention
The present invention relates to a cryogenic-fluid transfer line.
The invention relates more particularly to a cryogenic-fluid transfer line comprising a tubular outer jacket housing at least two inner fluid-transfer tubes and a heat screen forming an insulating wall positioned around the inner tubes, the outer jacket comprising a lateral pumping opening connected to a pumping member intended to form a vacuum in the outer jacket.
Related Art
Cryogenic-fluid transfer lines generally comprise an outer jacket housing, within its evacuated internal volume, one or more tubes (pipes) conveying fluid at cryogenic temperature (for example between 4 K and 80 K) and a thermal insulation system.
Documents FR2803898A1 and EP1020678A1 illustrate examples of details of cryogenic-fluid transfer lines.
The system for thermally insulating such lines generally comprises a heat screen (sometimes also referred to as a “cold screen”) comprising a conducting wall (made of aluminum or the like) arranged around the tube or tubes and cooled (“thermalized”) for example via contact with a cold tube (at 80 K for example).
The thermal insulation system also generally comprises multilayer thermal insulation arranged between the heat screen and the outer jacket.
The heat screen is centered in the outer jacket via separator members arranged at regular intervals along the entire length thereof and held on one of the tubes which is used for the thermalization thereof via points distributed along the length. Likewise, the fluid-transfer tube or tubes are held in place via fixed and sliding points arranged transversely in the outer jacket.
In order to create a vacuum in the outer jacket 2, the latter generally comprises a lateral pumping opening 5 connected to a pumping member 6.
The pumping thus draws out the gas:                from the space comprised between the multilayer insulation and the outer jacket,        through the multilayer insulation,        between the gaps in the heat screen (which gaps are caused by the fact that the structure of the heat screen is made up of portions assembled end to end).        
The pumping from the internal volume of the cold screen (where the tubes that carry the fluid are to be found) is therefore greatly slowed and limited by the pressure drops and conductance in the insulating layer and between the joints of the heat screen. This may have a strong influence on the ultimate thermal performance of the transfer line because the desired minimum pressure level is then not necessarily reached at certain points, and this is true whatever the pumping capability used.